1. Field of the Invention
This invention relates to apparatus for heating glass of ceramic vessels of the type commonly employed in electric cooking utensils known as slow cookers and, more particularly, to electric heating apparatus for uniformly heating the vessel.
2. Description of the Prior Art
An electric cooking utensil known as a slow cooker typically includes a glass or ceramic cooking vessel which is heated by a relatively low wattage heating element. In such appliances, cooking normally takes place at relatively low temperature over an extended period of time. The heat input to the glass or ceramic vessel can be provided in various ways. One approach is to wrap resistive heater wires directly around the vessel and to secure the wires to the vessel by cement or varnish. Another well-known design has resistive heater wires wrapped about a sheet of asbestos and sandwiched between two sheets of glass cloth, the complete assembly then being wrapped about the vessel. These approaches have the advantage of spreading the heat input across an extended portion of the outer surface of the vessel, the result being substantially uniform heating of the vessel and food contained in the vessel.
Sheath-type heating elements have many characteristics which would make their use advantageous for heating glass or ceramic cooking vessels. A sheath-type heating element includes an electrical resistance element which extends through an outer tubular protective metal sheath, and an electrical insulating compound such as compressed magnesium oxide is provided between the resistance element and the protective sheath to provide electrical isolation. It has not, however, been an easy matter to attach such heating elements to the outer surface of the cooking vessel. In addition, it would not be economically feasible to provide multiple turns of sheath-type heating elements about the cooking vessel; as a result, it has not heretofore been possible to provide uniform heating of the cooking vessel with sheath-type heating elements secured to the outer surface of the vessel.
It has been the practice to provide this type of cooking utensil with means of obtaining multiple heat input rates. Typically, this has been accomplished by providing an additional resistive heat element that can be operated in parallel with the primary heater to obtain a higher rate of heat input.